I don't think you will find Mariam Webster software made for Linux. There are different dictionaries you can install to make sure spell check and thesaurus functions work in word processors and other software you install though. And most distributions will provide at least one or two apps for finding word definitions, too.
A quick web search let me find a Webster's dictionary software package online, and (without trying it here) I'm pretty certain it wouldn't be hard to get that working under wine.